May 14, 2018

expert reaction to systematic review and meta-analysis on workplace physical activity and risk of early death

A new systematic review with meta-analysis, published in the British Journal of Sports Medicine, investigates whether highly physically active workers die early.

Prof Stephen Evans, Professor of Pharmacoepidemiology, London School of Hygiene and Tropical Medicine (LSHTM), said:

“This paper is in itself a well conducted systematic review, bringing together results from many different original studies of observational research on physical activity associated with employment.

“The original studies largely suffer from the possible lack of comparability of those in jobs with high, compared with low, physical activity. The authors themselves acknowledge this limitation, [“Certain lifestyle factors may or may not have been sufficiently adjusted for in the currently presented data”] though they seem to think that the more obvious explanation of a bias across most of the studies is not the reason for the supposed “paradox”.

“The classic study by Morris et al from 1953 (reference 9 from this paper) compared bus drivers with bus conductors. These had few differences in lifestyle (smoking, drinking etc) and social factors, and showed a strong effect of increased cardiovascular disease in those who were largely sedentary (drivers) while the conductors of London buses were continually climbing stairs to the upper deck and had less cardiovascular disease. This was a valid comparison, but many of the original studies in this review did not have the opportunity to make a valid comparison of a similar nature.

“It could well be that there are some forms of physical activity that are not beneficial, but it is not clear from this paper what they are.

“Certainly, this paper should not be used to suggest that exercise is not good for health.”

“It’s important to understand the role of occupational physical activity (PA) in relation to health outcomes, such as mortality and morbidity. However, the line of “Men with a highly physical job have an 18% higher risk of early death than those with inactive work” is misleading. On the basis of the current findings of this systematic review and meta-analysis the only conclusion one can draw is that there is an observed association of higher mortality rate between occupational PA of men from low socio-economic backgrounds, compared to men from other social backgrounds and less intensive occupational PA status.

“Despite the large sample size there are many methodological limitations of this study, including intensity of PA per day, over weeks and years spent in a job. Forty plus hours a week of repetitive activity in various environmentally hazardous conditions is not comparable to athletic training effects. The causes of death could also be related to occupational stress associated with little control, harsh working conditions, and an overall stress arising from socio-economic backgrounds of the current population. Typically, people in such jobs have higher social inequality profiles, indicating less access to paid holidays and sick leave, which threatens their job security if requested.

“In my view, at this stage there is no support for the ‘physical activity paradox’ based on this findings of this review.”

“Sedentary work compared to work that requires heavy physical activity is hugely confounded by education, social class and all the other associated behaviours.

“I think the difficulty with interpreting the findings of this review is that even though the authors stated that they included only studies that sufficiently adjusted for relevant factors, with sufficiently adjusted being defined as adjustment for age and gender, and at least one other relevant factor – including socioeconomic (e.g. education/income), lifestyle (e.g. smoking, alcohol use or leisure time PA) or health related factors (e.g. adiposity or blood pressure) – not all studies would have adjusted for some of the main factors that may strongly influence risk of early death so residual confounding with social class, education, health behaviours etc. cannot be excluded.

“It is quite possible that very heavy labour may be associated with adverse health. It may also be that these occupations lead to higher accident rates and early mortality without the physical activity itself being the relevant factor which the authors do discuss and I am sure that we need to understand this better.

“However, there is a substantial range of work place physical activity and in this respect, the definitions of high occupational activity and the comparisons made in the paper are important. From what I can see, the main analyses described aimed to compare the high category of occupational PA compared with the low category of occupational PA but not the in between categories so do not indicate what the associations are, for the in-between categories of occupational physical activity.

“This is relevant because most occupations that may involve some physical activity (plumbers, electricians, bus conductors, hair dressers) are much more prevalent that those that involve heavy physical activity such as dock yard workers, builders etc., and so the results do not address these occupations which have moderate physical activity. Additionally, the sex difference, in particular, the lack of increased mortality in women may well reflect, as the authors discuss, that women do not generally have occupations that require very heavy physical activity and/or the confounding is less marked.

“I think the challenge this review raises is whether we should revise current approaches which is to encourage more physical activity in the work place for those in generally sedentary jobs which is where most working people spend a substantial proportion of their lives. This paper, at least from my reading, does not provide evidence to change this approach as it only examines the relationship in the heavy physical activity end of the occupation spectrum.”

“These findings must look quite frightening for men whose job involves a lot of physical activity, but it’s important to dig into what the researchers actually found. For several reasons, it’s quite hard to interpret the findings, and you shouldn’t read too much into them. The researchers aren’t claiming that it must be the physical activity that actually causes the higher risk of early death. Their results are based on pooling information from observational studies, that is, studies that record the levels of physical activity in people’s jobs and then record any deaths that occur. The problem is that there will be many other differences between men in jobs with different levels of physical activity, apart from the level of physical activity, and perhaps it’s these other differences that actually lead to higher death rates in men with highly physical jobs. The researchers mention some possible other causes in their report – for instance, people in highly physical jobs tend to have lower socio-economic status, and other aspects of that status may lead to more early deaths. Also, some types of occupation both include lots of physical work and also have high death rates from accidents – the researchers mention construction and agriculture. It’s possible to adjust statistically for differences like this, and all the studies that were pooled did make some such adjustments, but studies can’t adjust for differences on which they don’t have data. Since all the studies that were pooled face the issue that they can’t produce clear evidence of cause and effect, the same issue has to exist when those studies are pooled.

“It’s very important to understand what was done statistically in this new research. The researchers combined the results of previous studies in an appropriate way, but they found high levels of heterogeneity between the studies that they pooled. That’s saying that the results from different studies just aren’t very consistent on whether risk of early death is higher in men in highly physical jobs – some studies found a lower risk of early death in those jobs – and even in those that did find a higher risk, the size of that higher risk differed quite a lot. You’d always expect some differences between studies, because of chance variation between the individuals they studied, but the high heterogeneity measures in this new research give strong evidence that the differences between studies are much bigger than can be explained by chance variation alone. That’s probably because the studies varied in important ways, in terms of the types of men they studied, the countries they lived in, the types of jobs they did, and so on. In other words, the individual studies that were pooled were, in a sense, addressing subtly different questions from one another. The headline figure of an 18% higher risk of death is an average across the findings on all these slightly different questions. If other research had been done with different groups of men or in different settings, the researchers would have had a different set of results to pool, and, because of the heterogeneity, they might well have got very different results. The researchers on the new study list several possible reasons for the heterogeneity, including differences between studies in the way they made statistical adjustments, and also including the possibility of publication bias, that is, failure to publish inconclusive research findings from small studies.

“Does all this messiness mean we can just ignore these new findings? Definitely not. The research certainly hasn’t shown that men doing physical jobs must cut back on their physical activity. But it clearly shows that we don’t know enough about the effects of workplace physical activity on health. It’s not adequate just to measure physical activity levels – research needs to look more thoroughly at the type of activity, the type of jobs, and wider characteristics of the people doing the jobs. That’s an important message for future researchers and for people drawing up guidelines on exercise.”

* ‘Do highly physically active workers die early? A systematic review with meta-analysis of data from 193 696 participants’ by Pieter Coenen et al. published in British Journal of Sports Medicine on Monday 14 May 2018.

Declared interests

Dr Erika Borkoles: No declarations of interest

Prof Kevin McConway: “I am a member of the Advisory Committee of the Science Media Centre.”